DIKES AND THEIR EFFECTS. 



stones which can be handled by a man without additional appliances, but it should be 

 covered and protected with stones which cannot be easily moved, and the heavier they 

 are the better for the stability of the dike. 



In the construction of a loose-stone dike a good stage of water is desirable in order 

 that boats may float over the site to deposit the stone, but the building of a crib dike 

 will not infrequently be facilitated by an ordinary stage. In this structure it is cus- 

 tomary to build the cribs in sections in the water immediately upon the site, or to tow 

 them to the location after they have been put together, when they are sunk upon the 

 river-bed and filled with stone or gravel, or both. As in the case of loose-stone dikes 

 it is advisable to place heavy stones on top or to deck with plank, the latter method 

 probably being more generally applied. 



The construction of pile dikes is very simple, consisting of little more than ordinary 

 pile-driving, and the dumping of loose stone from a scow. 



Closing Dikes. Rivers often divide into two or more passages. The effect of 

 this bifurcation is to diminish the flow; and to place the stream in a condition more 

 favorable to navigation at low-water stages it becomes necessary to close the additional 

 arms. This is generally accomplished by the construction of low dams or dikes across 

 the chutes, several being sometimes required in a single channel. The construction 

 of these dikes presents some special difficulties, not encountered in dikes along a shore. 

 Where built of stone the method is as follows: A layer of stones is placed upon the site 

 of the proposed work, extending sufficiently down stream to act as an apron to receive 

 and carry off the water overflowing the dike, and a sufficient distance up stream to form 

 the floor of the structure. A rule given by an authority * for the width of this bed of 

 stone is that it shall be fifteen times the fall produced. Its thickness must be deter- 

 mined with reference to the character of bed upon which it is placed, for when the 

 bottom is soft it is liable to sink. Upon the bed thus prepared the main structure is 

 built, in layers extending its entire length. The up-stream slope is about 3 of base to 

 2 of height, while that of the lower side is generally less, being about 2 to i for low 

 dikes, and decreasing as they become higher. As the construction proceeds the water 

 will rise over the structure and produce settlement at various points, or possibly along 

 the whole dike. Additional material must at once be put in in order that the profile 

 may be maintained at the desired elevation; otherwise a loss of some of the material 

 already in position may follow through undermining. As the water rises above the 

 dike its force increases up to a certain limit, after which the overflowing sheet loses its 

 scouring force. It is, therefore, advisable to proceed with great rapidity as the height 

 of the works is raised. The paving or coping should be of selected stones, hand-placed 

 and roughly jointed, in order that the surface presented to the overflow may be uniform 

 and solid. Piles can be used to good advantage in a foundation for this character of 

 structure, and effect a saving in stone. 



* De Mas. 



